Many times, owners, or contractors request if an excavation can be carried out without any structural support. Such cases often arise in construction projects involving smaller basements, cellars, and surface utility line excavations. While it may not be a common concern for deeper excavations such as deep basements or metro stations, it remains a critical decision that engineers and contractors must make. In such situations, the engineer's task is to demonstrate the feasibility of the construction, address potential issues and consequences, and ultimately make an informed engineering judgment that carries the responsibility to proceed or halt the unsupported excavation.
Is there a straightforward answer to this question?
Determining the feasibility of an unsupported excavation project is a complex process, contingent upon various factors. These factors include the characteristics of the existing soils and stratigraphy, the presence of nearby structures susceptible to soil settlements and displacements, the impact of external loads like buildings and roads on the stability of the vertical cut, and the need to protect adjacent utility lines sensitive to settlements. The project's duration and other variables also influence the overall stability of the soil mass, demanding careful consideration by the supervising engineer or contractor.
What criteria should engineers and contractors assess to make a safe decision?
Engineers should evaluate three key aspects:
A. Slope Stability Safety Factor: The initial step typically involves conducting a slope stability analysis using the Limit Equilibrium Method (LEM) or similar techniques. This analysis provides valuable insights into the likelihood of a land mass sliding within the excavation site—the primary concern in such scenarios. The stability of the slope hinges on factors such as soil properties and the presence and position of external loads like buildings and roads.
The following image depicts two distinct cases, one involving moderately cohesive soil (Friction angle 25 degrees, cohesion 180 psf (8.6 kPa)) and the other with highly cohesive soil (Friction angle 26 degrees, cohesion 600 psf (28.7 kPa)).
Case 1: The software computed a slope stability factor (FS) of 1.303, indicating a high risk of slope failure, as it falls below the typically desired minimum of 1.5 for failure surfaces passing through the adjacent structure. Some localized near the the vertical face wedges though might have smaller safety factors.
Case 2: The software computed an FS of 2.072, signifying a lower risk of a landslide, as factors above 1.5 suggest a low probability of failure, and factors above 2 indicate it is unlikely.
Figure 1: Slope stability analysis results for each examined scenario in DeepEX software
B. The soil mass settlements and total displacements. This aspect assumes great significance, especially when preexisting buildings and utility lines require protection. Allowable displacement limits can vary among global codes and regulations and may also rely on common-sense considerations. Inevitable settlements can lead to costly damage to adjacent properties. Engineers must anticipate expected displacements and associated damages through Finite Element Analysis (FEM), a method available in our shoring design software DeepEX. FEM accounts for soil-structure interaction, providing insights into soil horizontal displacements and settlements via shading and total displacement arrows.
The following images display surface settlement shadings and total displacement arrows for the two previously examined cases, considering an exponential behavior of both soils and modulus of elasticity values.
Case 1: Estimated settlements extend up to 0.7 inches (1.78cm) behind the excavation and beneath the adjacent building, with total displacement arrows indicating displacements up to 1.3 inches (3.3 cm) into the excavation. These are significant and may lead engineers to decline the excavation work.
Case 2: Estimated settlements are considerably smaller, reaching up to 0.06 inches (0.15 cm) behind the wall, and total displacement arrows show lower values, up to 0.13 inches (0.33 cm) behind the excavation—generally acceptable in engineering practice.
Figure 2: FEM soil mass settlements for each examined scenario in DeepEX software
Figure 3: FEM total displacement arrows for each examined scenario in DeepEX software
C. FEM Slope Stability with c’-φ’ Reduction. In this approach, the safety factor is determined by progressively reducing the shear strength of soil elements until the slope reaches a failure state. This method serves as a valuable FEM alternative to traditional slope stability analysis. Engineers should consider both approaches in delicate cases. While automatic algorithms may reveal complex slope stability mechanisms, they may not identify the most critical sliding surface. Conducting additional independent analyses can enhance confidence in the results.
The following image illustrates the strength reduction safety factor for each examined scenario. As expected, based on prior findings, the software calculates a safety factor of about 1 for the medium cohesive soil (Case 1). For the stronger soil (Case 2), the calculated safety factor from the c’-φ’ reduction approach is 2.17.
Figure 4: FEM c’-φ’ reduction safety factor for each examined scenario in DeepEX software
The decision and responsibility to design and construct unsupported excavations rest heavily on the shoulders of professional engineers. Even when an open excavation might be stable, we still must consider the possibility of adverse events such as rain caused erosion and water infiltration, or possibly seismic events. This article highlighted the diverse challenges associated with such decisions, highlighting potential issues that may arise. Additionally, we've demonstrated how powerful software solutions like DeepEX can facilitate quick and accurate evaluations of various scenarios, providing valuable insights into expected soil behavior. We encourage you to explore DeepEX and discover how it equips you with a comprehensive suite of analysis methods and options at your disposal for confident decision-making.